Treating of a crude containing natural gas

ABSTRACT

A process for treating a crude containing natural gas comprising supplying the crude to a stabilization unit to obtain a gaseous stream and crude oil; supplying a compressed, gaseous stream at a low temperature to the bottom of a first column; partly condensing the first gaseous overhead stream, returning the liquid phase to the first column and supplying the methane-rich stream to a liquefaction plant; supplying an expanded bottom stream at a low temperature to the top of a second column; removing from the top of the second column second gaseous overhead stream, and removing from the bottom of the second column a liquid bottom stream; vaporizing part of the bottom stream and introducing the vapour into the bottom of the second column; and introducing the remainder of the bottom stream into a crude oil stream at an appropriate point in or upstream of the stabilization unit, wherein the amount of heat removed from the first gaseous overhead stream is so adjusted that the concentration of C 5   +  in the first gaseous overhead stream is below a predetermined value, and wherein the fraction of the liquid bottom stream from the second column that is vaporized is so selected that the concentration of C 2   −  in the liquid bottom stream is below a predetermined level.

[0001] The present invention relates to treating a crude containingnatural gas. In the specification and in the claims the expression‘crude containing natural gas’ is used to refer to crude oil with whichnatural gas is produced. The natural gas is then called associated gas.In the specification and in the claims the expression ‘treating a crudecontaining natural gas’ is referred to treating the crude to obtain astabilized crude oil and a gaseous stream that can be passed directly toa liquefaction plant.

[0002] Stabilizing a crude oil in a stabilization unit is a well-knowntechnique to produce stabilized crude oil and a gaseous stream. Suitablythe stabilization unit is a multi-stage separator.

[0003] In addition it is well known to remove a C₂ ⁺-containing streamfrom the gaseous stream to obtain a gaseous stream that is enriched inmethane and that can be passed directly to a liquefaction plant and a C₂⁺-containing stream. The C₂ ⁺-containing stream can be used as fuel gas,to produce liquefied petroleum gas or to provide components for therefrigerants used in the liquefaction plant.

[0004] Reference is made to USA patent specification U.S. Pat. No.5,030,339. This publication discloses a process for treating a crudecontaining natural gas, which process comprises:

[0005] (a) supplying the crude to a stabilization unit to obtain agaseous stream, a first condensate stream and crude oil;

[0006] (b) partly condensing the gaseous stream at an elevated pressurein a refluxing heat exchanger to obtain export gas and a secondcondensate stream;

[0007] (c) combining the first and second condensate streams; allowingthe combined condensate stream to expand and heating the condensatestream;

[0008] (d) supplying the expanded condensate stream to a column;

[0009] (e) removing from the top of the column a fuel gas stream, andremoving from the bottom of the column a liquid bottom stream;

[0010] (f) vaporizing part of the liquid bottom stream and introducingthe vapour into the bottom of the column; and

[0011] (g) combining the remainder of the liquid bottom stream with thecrude oil stream.

[0012] There are instances where it is desirable to minimize the fuelgas stream, without sacrificing the requirement of producing astabilized crude oil and producing a methane-rich stream that can bepassed directly to a liquefaction plant. In addition, there is a need tomeet more stringent requirements on the concentrations of C₅ ⁺ in theproduct gas of and C₂ ⁻ in the crude oil.

[0013] To this end the process for treating a crude containing naturalgas according to the present invention comprises:

[0014] (a) supplying the crude to a stabilization unit to obtain agaseous stream and crude oil;

[0015] (b) cooling the gaseous stream at an elevated pressure andsupplying the gaseous stream at a low temperature to the bottom of afirst column;

[0016] (c) removing from the top of the first column a first gaseousoverhead stream, partly condensing the gaseous overhead stream,separating the liquid phase from the partly condensed overhead stream toobtain a methane-rich stream, returning the liquid phase to the top ofthe first column and supplying the methane-rich stream to a liquefactionplant;

[0017] (d) removing a bottom stream from the first column, allowing thebottom stream to expand to a lower pressure, and supplying the expandedbottom stream at a low temperature to the top of a second column;

[0018] (e) removing from the top of the second column a second gaseousoverhead, and removing from the bottom of the second column a liquidbottom stream;

[0019] (f) vaporizing part of the liquid bottom stream and introducingthe vapour into the bottom of the second column; and

[0020] (g) introducing the remainder of the liquid bottom stream into acrude oil stream at an appropriate point in or upstream of thestabilization unit, wherein the amount of heat removed from the firstgaseous overhead stream is so adjusted that the concentration of C₅ ⁺ inthe first gaseous overhead stream is below a predetermined value, andwherein the fraction of the liquid bottom stream from the second columnthat is vaporized is so selected that the concentration of C₂ ⁻ in theliquid bottom stream is below a predetermined level.

[0021] An advantage of the process of the present invention is that itcan be used on a floating system for producing liquefied natural gasbecause the C₂ ⁺-containing stream is not produced as a separate stream.Thus there is no need for separate off-loading facilities for liquefiedpetroleum gas, which simplifies the floating system.

[0022] The invention will now be discussed by way of example in moredetails with reference to the accompanying drawing showing schematicallya flow sheet of the plant in which the method of the present inventioncan be carried out.

[0023] The plant comprises a stabilization unit 3 and a fractionationunit 4. The crude containing natural gas is supplied through conduit 6to the stabilization unit 3. Stabilized crude oil is removed from thestabilization unit 3 through conduit 8, and a gaseous stream is removedfrom the stabilization unit 3 through a gas-collecting conduit 10.

[0024] The gaseous stream removed through the gas-collecting conduit 10is supplied to the fractionation unit 4, and a methane-rich stream isremoved from the fractionation unit 4 through conduit 14. Thismethane-rich stream is supplied to a plant for liquefying natural gas(not shown).

[0025] We will first discuss the stabilization unit 3 and then we willdiscuss the fractionation unit 4.

[0026] The crude containing natural gas supplied through the conduit 6is supplied to a high-pressure separator 20. A high-pressure gaseousstream is withdrawn from the high-pressure separator 20 through thegas-collecting conduit 10, and oil is removed through conduit 21. Ifwater is present in the crude, it is removed through conduit 22. The oilremoved through conduit 21 is passed through an optional heater 23 and apressure-reduction valve 25 to a low-pressure separator 26. Alow-pressure gas stream is withdrawn from the low-pressure separator 26through conduit 27, and before it is introduced in the gas-collectingconduit 10 its pressure is increased using compressor 28. Oil is removedfrom the low-pressure separator 26 through conduit 30. The oil is passedvia an optional oil/water separator 31 through conduit 32 provided witha pressure-reduction valve 33 to an atmospheric separator 35. Water isremoved from the optional oil/water separator 31 through conduit 37. Theatmospheric separator 35 is the last separator of the stabilization unit3, and from the atmospheric separator 35 stabilized oil is withdrawnthrough conduit 8 and a gaseous stream is withdrawn through conduit 38,and before it is introduced in the gas-collecting conduit 10 itspressure is increased using compressor 39.

[0027] Now the fractionation unit 4 is discussed in more detail. Thegaseous stream supplied through conduit 10 is brought to an elevatedpressure—if necessary—by compressor 40, and at elevated pressure thegaseous stream is supplied through conduit 42 to a heat exchanger 43, inwhich it is cooled to a low temperature by indirect heat exchange with asuitable refrigerant. The refrigerant is suitably a refrigerant that isalso used in the liquefaction plant. The gaseous stream is supplied atlow temperature through conduit 46 to the bottom of a first column 47provided with a suitable number of theoretical separation stages 48.Suitably the number of theoretical separation stages 48 is in the rangeof from 10 to 30. From the first column 47 a first gaseous overheadstream is removed through conduit 50, which first gaseous overheadstream is supplied to a heat exchanger 53, in which it is cooled to alow temperature by indirect heat exchange with a suitable refrigerant soas to partly condense the gaseous overhead stream. The refrigerant issuitably a refrigerant that is also used in the liquefaction plant. Thepartly condensed gaseous overhead stream is supplied through conduit 55to a gas/liquid separator 56 to obtain a methane-rich stream that issupplied through conduit 14 to the plant for liquefying this gas. Theliquid phase is returned through conduit 58 from the gas/liquidseparator 56 to the first column 47.

[0028] The first column 47 is a rectifying column operating at fullreflux conditions. The amount of heat removed from the gaseous overheadstream is so adjusted that the concentration of C₅ ⁺ in the gaseousoverhead stream from the first column 47 is below a predetermined value.Suitably the concentration of C₅ ⁺ is less than 0.1 mol %.

[0029] The bottom stream removed from the first column 47 is passedthrough conduit 60 provided with a pressure reduction valve 63 at a lowtemperature directly to the top of a second column 65 provided with asuitable number of theoretical separation stages 66. Suitably the numberof theoretical separation stages 48 is in the range of from 10 to 30.From the top of the second column 65 a second gaseous overhead isremoved through conduit 67. The second gaseous overhead of the secondcolumn 65 can be used as fuel gas. From the bottom of the second column65 a liquid bottom stream is removed, wherein part of the liquid bottomstream is returned to the bottom of the second column 65 through conduit69 provided with a reboiler 73 to vaporize that part of the liquidbottom stream. The remainder of the liquid bottom stream is introducedinto a crude oil stream at an appropriate point in or upstream of thestabilization unit 4. In the embodiment shown in the drawing theremainder is passed through conduit 75 and mixed with the oil in conduit32.

[0030] The second column 65 is a stripping column operating at lowerpressure than the first column 47. The fraction of the liquid bottomstream of the second column 65 that is vaporized is so selected that theconcentration of C₂ ⁻ in the liquid bottom stream is below apredetermined level. Suitably the C₂ ⁻ concentration is below between 1mol % and more suitably below 0.2 mol %.

[0031] In summary the essence of the present invention resides in thefollowing features, stabilizing the crude in a known stabilization unit3, operating the first column 47 at an elevated pressure, controllingthe reflux of the first column 47 so that the gaseous overhead is sorich in methane that it can be passed directly to a liquefaction plant,operating the second column 65 at a lower pressure, controlling thereboiling of the second column 65 such that the liquid bottom stream hasa sufficiently low C₂ ⁻-content, and mixing the liquid bottom streamwith crude oil upstream the fractionation unit 4. On its own the liquidbottom stream removed from the second column 75 is not stable atatmospheric conditions, but the mixture of this stream with the crudeoil is stable because the lighter hydrocarbons will dissolve in thecrude oil.

[0032] The stabilization unit discussed with reference to the drawinghas three separation stages for separating gas and liquid, which are theseparators 20, 26 and 35. However, any suitable number of separationstages can be employed, depending on the particular crude that is to betreated. The conditions in the stabilization unit are known and will notbe discussed in more detail.

[0033] Suitably, the elevated pressure in the first column 47 is in therange of from 4 to 7 MPa, and the low temperature of the gaseous streamthat is supplied through conduit 46 is in the range of from −10 to −20°C.

[0034] Suitably the expanded bottom stream is supplied to the top of asecond column 65 at a temperature that is below the low temperature ofthe cooled gaseous stream, and more suitably, this temperature is in therange of from −20 to −40° C. Suitably the lower pressure with which thebottom stream from the first column 47 is supplied to the top of thesecond column 65 is in the range of from 2.5 to 3 MPa.

[0035] In the embodiment shown in the drawing, the liquid bottom streamfrom the second column 65 is introduced into the crude oil streambetween the second and third separation stage of stabilization unit 3.The appropriate point at which this remainder can be introduced into thecrude oil stream can be in conduit 6, or in between any of theseparation stages.

[0036] The gas that is passed through conduit 10 to the fractionationunit 4 is suitably treated upstream of the fractionation unit 4. Thetreatment includes removing contaminants such as carbon dioxide from thegas, and drying the gas. The treating units have not been shown in thedrawing.

1. A process for treating a crude containing natural gas, which processcomprises: (a) supplying the crude to a stabilization unit to obtain agaseous stream and crude oil; (b) cooling the gaseous stream at anelevated pressure and supplying the gaseous stream at a low temperatureto the bottom of a first column; (c) removing from the top of the firstcolumn a first gaseous overhead stream, partly condensing the gaseousoverhead stream, separating the liquid phase from the partly condensedoverhead stream to obtain a methane-rich stream, returning the liquidphase to the top of the first column and supplying the methane-richstream to a liquefaction plant; (d) removing a bottom stream from thefirst column, allowing the bottom stream to expand to a lower pressure,and supplying the expanded bottom stream at a low temperature to the topof a second column; (e) removing from the top of the second column asecond gaseous overhead stream, and removing from the bottom of thesecond column a liquid bottom stream; (f) vaporizing part of the liquidbottom stream and introducing the vapour into the bottom of the secondcolumn; and (g) introducing the remainder of the liquid bottom streaminto a crude oil stream at an appropriate point in or upstream of thestabilization unit, wherein the amount of heat removed from the firstgaseous overhead stream is so adjusted that the concentration of C₅ ⁺ inthe first gaseous overhead stream is below a predetermined value, andwherein the fraction of the liquid bottom stream from the second columnthat is vaporized is so selected that the concentration of C₂ ⁻ in theliquid bottom stream is below a predetermined level.
 2. The process ofclaim 11, wherein the elevated pressure in step (b) is in the range offrom 4 to 7 MPa, and wherein the low temperature is in the range of from−10 to −20° C.
 3. The process of claim 1, wherein the expanded bottomstream in step (d) is supplied to the top of a second column at atemperature that is below the low temperature in step (b).
 4. Theprocess of claims 1 wherein the lower pressure in step (d) is in therange of from 2.5 to 3 MPa, and wherein the low temperature is in therange of from −20 to 40° C.
 5. The process of claim 2 wherein theexpanded bottom stream in step (d) is supplied to the top of a secondcolumn at a temperature that is below the low temperature in step (b).6. The process of claim 2 wherein the lower pressure in step (d) is inthe range of from 2.5 to 3 MPa, and wherein the low temperature is inthe range of from −20 to 40° C.
 7. The process of claim 3 wherein thelower pressure in step (d) is in the range of from 2.5 to 3 MPa, andwherein the low temperature is in the range of from −20 to 40° C.